In telephone subsets there are mostly used acoustic high impedance transducers. This type of transducer is a fully closed unit with no connections to the inner space of the handset house. When using a high impedance transducer in a telephone subset there will be no or only very little reproduction of the bass range (the low frequencies) and the reproduced voice or music has a metallic sound. The reason for this missing bass reproduction is that when using a telephone there will always be a leak between the handset and the ear (there is no hermetic closure) and this even small leakage will effectively remove the whole bass range. When telephone handsets are tested there is used an artificial ear which is hermetically connected to the telephone handset, whereby the measurements show the correct bass content. There is in effect no or only very little correspondence between the measured and the real situation for the bass range.
The invention relates in particular to telephone handsets using transducers of the acoustic low impedance type. An acoustic low impedance transducer is not a closed unit and makes use of the space which is available behind the transducer. The purpose of using an acoustic low impedance transducer is to obtain a better and more natural reproduction of sound. The acoustic low impedance transducer can tolerate a little leakage between the ear and the telephone handset without losing the bass range.
The acoustic low impedance transducers are used in hi-fi headphones where the situation is different from telephone sets. Headphones have an open structure, whereas it is desirable that a telephone handset is closed in order to prevent the speech of the transmitting subscriber from being overheard by a third person in the receiving end and shielding the ear from surrounding noise.